Pierre Y. Bernier

Boreal forest health and global change.

The boreal forest, one of the largest biomes on Earth, provides ecosystem services that benefit society at levels ranging from local to global. Currently, about two-thirds of the area covered by this biome is under some form of management, mostly for wood production. Services such as climate regulation are also provided by both the unmanaged and managed boreal forests. Although most of the boreal forests have retained the resilience to cope with current disturbances, projected environmental changes of unprecedented speed and amplitude pose a substantial threat to their health. Management options to reduce these threats are available and could be implemented, but economic incentives and a greater focus on the boreal biome in international fora are needed to support further adaptation and mitigation actions.

FOREST PRODUCTIVITY DECLINE CAUSED BY SUCCESSIONAL PALUDIFICATION OF BOREAL SOILS

Long-term forest productivity decline in boreal forests has been extensively studied in the last decades, yet its causes are still unclear. Soil conditions associated with soil organic matter accumulation are thought to be responsible for site productivity decline. The objectives of this study were to determine if paludification of boreal soils resulted in reduced forest productivity, and to identify changes in the physical and chemical properties of soils associated with reduction in productivity. We used a chronosequence of 23 black spruce stands ranging in postfire age from 50 to 2350 years and calculated three different stand productivity indices, including site index. We assessed changes in forest productivity with time using two complementary approaches: (1) by comparing productivity among the chronosequence stands and (2) by comparing the productivity of successive cohorts of trees within the same stands to determine the influence of time independently of other site factors. Charcoal stratigraphy indicates that the forest stands differ in their fire history and originated either from high- or low-severity soil burns. Both chronosequence and cohort approaches demonstrate declines in black spruce productivity of 50-80% with increased paludification, particularly during the first centuries after fire. Paludification alters bryophyte abundance and succession, increases soil moisture, reduces soil temperature and nutrient availability, and alters the vertical distribution of roots. Low-severity soil burns significantly accelerate rates of paludification and productivity decline compared with high-severity fires and ultimately reduce nutrient content in black spruce needles. The two combined approaches indicate that paludification can be driven by forest succession only, independently of site factors such as position on slope. This successional paludification contrasts with edaphic paludification, where topography and drainage primarily control the extent and rate of paludification. At the landscape scale, the fire regime (frequency and severity) controls paludification and forest productivity through its effect on soil organic layers. Implications for global carbon budgets and sustainable forestry are discussed.

Validation of a canopy gas exchange model and derivation of a soil water modifier for transpiration for sugar maple (Acer saccharum Marsh.) using sap flow density measurements

Abstract Hourly plot-level transpiration measurements were carried out in a stand of sugar maple (Acer saccharum Marsh.) near Quebec City, Canada, during one summer using radial flow meters installed in selected trees. The measurements were used to validate transpiration estimates obtained from a multi-layer gas exchange model that included the modeling of the within-canopy radiation regime and the thermal balance of the leaves. The comparison between modeled and measured hourly transpiration showed no bias in the predictions, and an r2 value of 0.80. Because of the close coupling between transpiration and photosynthesis built in the model, these results suggest that modeled estimates of photosynthesis should also be well-related to actual rates at the stand-level. The transpiration data were also used to adjust an empirical transpiration model based on the Penman–Monteith equation in which the canopy conductance term was replaced by a function of global radiation, vapor pressure deficit and soil water depletion. This empirically-fitted model captured 85% of the variation observed in the data, including the effect of soil water depletion during a late-season drought. The soil water modifier included within this equation was compared with other soil water modifier functions obtained from the literature. The comparison highlights two difficulties in the derivation of soil water modifiers applicable outside experimental areas. The first is in the determination of rooting depth so that mass balances of soil water content can be carried out. The second is in the determination of soil physical properties so that absolute values of moisture contents can be translated into relative values of water availability.

Negative impacts of high temperatures on growth of black spruce forests intensify with the anticipated climate warming

An increasing number of studies conclude that water limitations and heat stress may hinder the capacity of black spruce (Picea mariana (Mill.) B.S.P.) trees, a dominant species of Canadas boreal forests, to grow and assimilate atmospheric carbon. However, there is currently no scientific consensus on the future of these forests over the next century in the context of widespread climate warming. The large spatial extent of black spruce forests across the Canadian boreal forest and associated variability in climate, demography, and site conditions pose challenges for projecting future climate change responses. Here we provide an evaluation of the impacts of climate warming and drying, as well as increasing [CO2 ], on the aboveground productivity of black spruce forests across Canada south of 60°N for the period 1971 to 2100. We use a new extensive network of tree-ring data obtained from Canadas National Forest Inventory, spatially explicit simulations of net primary productivity (NPP) and its drivers, and multivariate statistical modeling. We found that soil water availability is a significant driver of black spruce interannual variability in productivity across broad areas of the western to eastern Canadian boreal forest. Interannual variability in productivity was also found to be driven by autotrophic respiration in the warmest regions. In most regions, the impacts of soil water availability and respiration on interannual variability in productivity occurred during the phase of carbohydrate accumulation the year preceding tree-ring formation. Results from projections suggest an increase in the importance of soil water availability and respiration as limiting factors on NPP over the next century due to warming, but this response may vary to the extent that other factors such as carbon dioxide fertilization, and respiration acclimation to high temperature, contribute to dampening these limitations.

Integrated forestry assessments for climate change impacts

Forests and the forest sector are sensitive to climate change at greatly varying scales. The complexity of the interactions among the physical environment, forest growth, the management and utilisation of forest resources, and market responses has stimulated efforts to model the impact of global changes on the forest sector by linking impact models developed from different disciplines. This paper reviews existing experiences in integrated forest sector impact assessments. Different ways of integrating cross-disciplinary impact assessments are classified as linking, coupling and integrated modelling. To date the most common method is a “one-way” linking, where results from one model are used as input to a different model. When different impact models are coupled, feedbacks can be analysed, e.g. between ecological and economic systems. Integrated modelling is described as a third step, where different sub-models are embedded into a common model framework. The concept of balance is introduced as a key to successful integration of different disciplines in integrated assessment (IA) studies. The review of existing experiences emphasises the problem of complexity and the need to simplify disciplinary approaches. It also illustrates how methodologies applied to forest sector IA studies have evolved over the last few years. Several scaling issues that are particularly important for IA modelling in forestry are discussed, including the consequences of heterogeneity in site conditions, the variable influence of extreme events on ecosystems and on the economic sector, and the differences in temporal and spatial scales over which key forest growth and renewal processes operate. Climate impact assessments include uncertainties. Some common sources of uncertainty in forest IA modelling are outlined, and methods that have been used to address this uncertainty are reviewed. We discuss the policy relevance of integrated impact assessments and stress the importance of stakeholder involvement in IA projects. The paper concludes with some recommendations for future developments in this relatively new field of research.

Initial size and competing vegetation effects on water stress and growth of Picea mariana (Mill.) BSP seedlings planted in three different environments

Abstract Three experimental sites in Quebec were planted with four different sizes of containerized black spruce seedlings (Picea mariana (Mill.) BSP). We examined the water stress experienced by each stock size of black spruce seedlings in relation to different competing vegetation covers and also the effect of the water stress on spruce growth during the first three growing seasons. The sites consisted of one abandoned agricultural field and two forest locations. Containers of sizes 45–110, 45–340, 15–700, and 12–1000 were employed to produce the four different sizes of spruce seedlings. At each site, the experimental protocol used a split-plot in a randomized complete block design, in which the presence of a competing vegetation cover (weedy and bare plots) was assigned to the whole plot, while a specific seedling size was assigned to each subplot. At each experimental site both the predawn xylem water potential Ψxp and the midday value Ψxm were measured three times during each of the first three growing seasons. Data were analysed as a completely randomized split-split-plot design, where selection of seedlings in time was considered as the whole plot. The competing vegetation tended to protect the spruce seedlings from excessive water loss, without depressing the soil-water potential (SWP) to the point of reducing the moisture available to the seedlings. Both Ψxp and Ψxm were found to decrease significantly with increasing initial seedling size. The increased water stress experienced by the large stock of spruce seedlings had an effect on the absolute growth rate (AGR) in height on only one experimental site. The AGR was impaired by the presence of a competing vegetation cover, but more severely for the smaller stock-size than the larger. The short-term effect of a competition should be based on radial growth; height growth and mortality are not early indicators of such effect. These results emphasize the need to produce a large stock of spruce seedlings with well-developed root systems and root growth capacity, even though only moderate water stress was observed during the first three years of plantation growth.

Changes in net ecosystem productivity of boreal black spruce stands in response to changes in temperature at diurnal and seasonal time scales.

Net ecosystem productivity (NEP) of boreal coniferous forests is believed to rise with climate warming, thereby offsetting some of the rise in atmospheric CO(2) concentration (C(a)) by which warming is caused. However, the response of conifer NEP to warming may vary seasonally, with rises in spring and declines in summer. To gain more insight into this response, we compared changes in CO(2) exchange measured by eddy covariance and simulated by the ecosystem process model ecosys under rising mean annual air temperatures (T(a)) during 2004-2006 at black spruce stands in Saskatchewan, Manitoba and Quebec. Hourly net CO(2) uptake was found to rise with warming at T(a) < 15 degrees C and to decline with warming at T(a) > 20 degrees C. As mean annual T(a) rose from 2004 to 2006, increases in net CO(2) uptake with warming at lower T(a) were greater than declines with warming at higher T(a) so that annual gross primary productivity and hence NEP increased. Increases in net CO(2) uptake measured at lower T(a) were explained in the model by earlier recovery of photosynthetic capacity in spring, and by increases in carboxylation activity, using parameters for the Arrhenius temperature functions of key carboxylation processes derived from independent experiments. Declines in net CO(2) uptake measured at higher T(a) were explained in the model by sharp declines in mid-afternoon canopy stomatal conductance (g(c)) under higher vapor pressure deficits (D). These declines were modeled from a hydraulic constraint to water uptake imposed by low axial conductivity of conifer roots and boles that forced declines in canopy water potential (psi(c)), and hence in g(c) under higher D when equilibrating water uptake with transpiration. In a model sensitivity study, the contrasting responses of net CO(2) uptake to specified rises in T(a) caused annual NEP of black spruce in the model to rise with increases in T(a) of up to 6 degrees C, but to decline with further increases at mid-continental sites with lower precipitation. However, these contrasting responses to warming also indicate that rises in NEP with climate warming would depend on the seasonality (spring versus summer) as well as the magnitude of rises in T(a).

Paludification dynamics in the boreal forest of the James Bay Lowlands: effect of time since fire and topography.

In many northern forest ecosystems, soil organic matter accumulation can lead to paludification and forest pro- ductivity losses. Paludification rate is primarily influenced by topography and time elapsed since fire, two factors whose in- fluence is often confounded and whose discrimination would help forest management. This study, which was conducted in the black spruce (Picea mariana (Mill.) BSP) boreal forest of northwestern Quebec (Canada), aimed to (1) quantify the ef- fect of slope and time since fire on paludification rates, (2) determine whether soil organic layer depth could be estimated by surface variables that can potentially be remotely sensed, and (3) relate the degree of paludification to tree productivity. In this study, soil organic layer depth was used as an estimator of the degree of paludification. Slope and postfire age strongly affected paludification dynamics. Young stands growing on steep slopes had thinner organic layers and lower or- ganic matter accumulation rates compared with young stands growing on flat sites. Black spruce basal area and Sphagnum cover were strong predictors of organic layer depth, potentially allowing mapping of paludification degree across the land- scape. Tree productivity was negatively related to organic layer depth (R 2 = 0.57). The equations developed here can be used to quantify forest productivity decline in stands that are undergoing paludification, as well as potential productivity re- covery given appropriate site preparation techniques.

A plane intersect method for estimating fine root productivity of trees from minirhizotron images

The advent of minirhizotrons more than a decade ago has made the careful and widespread study of fine root dynamics of trees possible. However, to this day, the estimation of fine root productivity in terms of mass production per unit of ground surface from the minirhizotron data remains hampered by the difficulty in transforming images of roots captured along a two-dimensional plane into estimates of root volume or mass within a soil volume. In this work, we propose that the date of fine root appearance and the diameter of fine roots are the most robust variables that can be obtained from minirhizotron measurements of tree roots and that these two variables should be the basis of productivity estimates. The method proposed for estimating fine root productivity expands the line intersect method of Van Wagner (1968) into a plane intersect method that permits, with the appropriate volumetric transformations and corrections for tube and slope angles, the estimation of fine root productivity per unit ground area for specific periods. Examples of calculations are presented for two datasets obtained within two different forested sites, as well as a comparison with a methodology based on camera depth-of-view. The main weakness of the plane intersect method is the assumption that all fine root segments are independent. The correction for the fraction of coarse particles also creates uncertainty in the final estimate.

Effect of shoot size on the gas exchange and growth of containerized Picea mariana seedlings under different watering regimes

Containerized black spruce (Picea mariana [Mill.] B.S.P.) seedlings of three different sizes (small, medium, and large) were planted in raised sand beds maintained under wet, moderately dry or dry watering regimes during the growing season. The small seedlings were of a conventional stock type. The two larger sizes were novel stock types grown in larger containers. Physiological measurements during the summer showed that the small and medium seedlings maintained nearly similar levels of gas exchanges and water status, but that the large seedlings had reduced net photosynthesis and stomatal conductance under all watering regimes. Analysis of dry masses showed comparable relative growth rates in the small and medium seedlings, but a small to null growth in the large seedlings. Examination of root relative growth rate under wet conditions revealed significant root growth in small and medium seedlings, but negligible growth in the large seedlings. It was concluded that increasing the shoot size of containerized seedlings can be achieved without increasing the susceptibility of the seedlings to water stress, as long as the vigour of the root system is maintained.